The presence of Greeks on the Italian peninsula from the beginning of the historical period influenced Roman culture, introducing some religious practices that became as fundamental as the cult of Apollo. The Romans looked for common ground between their major gods and those of the Greeks (interpretatio graeca), adapting Greek myths and iconography for Latin literature and Roman art, as the Etruscans had. Etruscan religion was also a major influence, particularly on the practice of augury. According to legends, most of Rome's religious institutions could be traced to its founders, particularly Numa Pompilius, the Sabine second king of Rome, who negotiated directly with the gods. This archaic religion was the foundation of the mos maiorum, "the way of the ancestors" or simply "tradition", viewed as central to Roman identity.
Roman religion was practical and contractual, based on the principle of do ut des, "I give that you might give". Religion depended on knowledge and the correct practice of prayer, ritual, and sacrifice, not on faith or dogma, although Latin literature preserves learned speculation on the nature of the divine and its relation to human affairs. Even the most skeptical among Rome's intellectual elite such as Cicero, who was an augur, saw religion as a source of social order. As the Roman Empire expanded, migrants to the capital brought their local cults, many of which became popular among Italians. Christianity was in the end the most successful of these, and in 380 became the official state religion.
For ordinary Romans, religion was a part of daily life. Each home had a household shrine at which prayers and libations to the family's domestic deities were offered. Neighborhood shrines and sacred places such as springs and groves dotted the city. The Roman calendar was structured around religious observances. Women, slaves, and children all participated in a range of religious activities. Some public rituals could be conducted only by women, and women formed what is perhaps Rome's most famous priesthood, the state-supported Vestals, who tended Rome's sacred hearth for centuries, until disbanded under Christian domination.
The priesthoods of public religion were held by members of the elite classes. There was no principle analogous to separation of church and state in ancient Rome. During the Roman Republic (509–27 BC), the same men who were elected public officials might also serve as augurs and pontiffs. Priests married, raised families, and led politically active lives. Julius Caesar became pontifex maximus before he was elected consul.
The augurs read the will of the gods and supervised the marking of boundaries as a reflection of universal order, thus sanctioning Roman expansionism as a matter of divine destiny. The Roman triumph was at its core a religious procession in which the victorious general displayed his piety and his willingness to serve the public good by dedicating a portion of his spoils to the gods, especially Jupiter, who embodied just rule. As a result of the Punic Wars (264–146 BC), when Rome struggled to establish itself as a dominant power, many new temples were built by magistrates in fulfillment of a vow to a deity for assuring their military success.
As the Romans extended their dominance throughout the Mediterranean world, their policy in general was to absorb the deities and cults of other peoples rather than try to eradicate them, since they believed that preserving tradition promoted social stability. One way that Rome incorporated diverse peoples was by supporting their religious heritage, building temples to local deities that framed their theology within the hierarchy of Roman religion. Inscriptions throughout the Empire record the side-by-side worship of local and Roman deities, including dedications made by Romans to local gods.
By the height of the Empire, numerous international deities were cultivated at Rome and had been carried to even the most remote provinces, among them Cybele, Isis, Epona, and gods of solar monism such as Mithras and Sol Invictus, found as far north as Roman Britain. Foreign religions increasingly attracted devotees among Romans, who increasingly had ancestry from elsewhere in the Empire. Imported mystery religions, which offered initiates salvation in the afterlife, were a matter of personal choice for an individual, practiced in addition to carrying on one's family rites and participating in public religion. The mysteries, however, involved exclusive oaths and secrecy, conditions that conservative Romans viewed with suspicion as characteristic of "magic", conspiratorial (coniuratio), or subversive activity. Sporadic and sometimes brutal attempts were made to suppress religionists who seemed to threaten traditional morality and unity, as with the senate's efforts to restrict the Bacchanals in 186 BC. Because Romans had never been obligated to cultivate one god or one cult only, religious tolerance was not an issue in the sense that it is for competing monotheistic systems. The monotheistic rigor of Judaism posed difficulties for Roman policy that led at times to compromise and the granting of special exemptions, but sometimes to intractable conflict. For example, religious disputes helped cause the First Jewish–Roman War and the Bar Kokhba revolt.